1. Field of the Invention
The present invention relates to a liquid crystal display device. In particular, the invention relates to a liquid crystal display device mounting a polarizing plate using an optical compensation film contributive to improvement of a front contrast ratio of the liquid crystal display, a polarizer and a polarizing plate protective film.
2. Description of Related Art
A liquid crystal display device is a space-saving image display device having low power consumption and the uses thereof are expanded every year. In the background art, the high dependence of an image on a viewing angle is a serious weakness of the liquid crystal display device. However, a high viewing angle liquid crystal mode by a VA (Vertical Alignment) mode has been recently commercialized. As a result, the demands for the liquid crystal display device are rapidly spreading even in markets in which an image of high definition is required such as TV.
The VA mode liquid crystal display device has a merit that it has generally high contrast ratio, compared to the other liquid crystal display modes. However, further improvements are required.
In order to obtain the high contrast ratio, it is examined to lower brightness at the time of the black display. When it is desired to lower the brightness at the time of black display, it has only to use a film having low haze properties. However, when a surface haze is lowered too much, the film is closely adhered or scratch is easily formed on a surface thereof.
In addition, it is known that it is necessary to lower an internal haze of a phase difference film for improvement of the front contrast ratio (JP-A-9-80424). However, since the front contrast ratio of a panel is low such as 1000 or less in the background art, it is not necessary to control even a refractive index of a member constituting the liquid crystal display device. However, the front contrast ratio (CR) of the panel is recently improved in the VA mode liquid crystal display (CR=5000 to 10000) and it is thus necessary to control a refractive index of a polarizing plate.
As a means for improving a tone depending on the optic angle or a viewing angle of the black display, it is investigated even in an IPS or FFS mode to arrange an optical compensation material having a birefringence characteristic between a liquid crystal layer and a polarizing plate. For example, it is disclosed that a birefringence medium, in which optic axes having a function of compensating an increase or decrease in retardation of the liquid crystal layer at the time of slanting are made to be orthogonal to each other, is arranged between a substrate and the polarizing plate, thereby improving coloring when directly viewing the white display or halftone display from an oblique direction (refer to JP-A-9-80424). However, the front contrast ratio is not sufficiently improved by the method.
In addition, when an optical film such as antireflection film or conductive film (ITO film) is stacked to a display panel such as liquid crystal display device, adhesive composition including acrylic-based polymer is used. However, in the background art, since a refractive index of the adhesive composition after drying and/or curing of the composition is lower than those of the optical films, a difference of the refractive indices is caused to disturb the effective using of light. Regarding this, JP-A-2002-173656 discloses of using a specific ring-containing copolymerizable monomer component having a high refractive index after drying and/or curing of composition as a monomer unit of pressure sensitive adhesive composition so as to obtain a pressure sensitive adhesive having a refractive index close to that of an optical film or optical member.
Thus, an optical film is disclosed which has a purpose of reducing total reflection at each interface of optical film/pressure sensitive adhesive layer/optical member to effectively use the light.
Furthermore, JP-A-2007-279083 discloses a plastic film in which inner and surface hazes are adjusted to specific ranges so as to improve productivity such as handling and preservation properties of the film in a roll state and front contrast ratio of a liquid crystal display device, particularly IPS-type liquid crystal display device and in-plane retardation and thickness-direction retardation are adjusted to specific ranges so as to improve a viewing angel characteristic of the liquid crystal display device.
However, JP-A-9-80424 does not disclose that the front contrast ratio is remarkably improved by adapting the refractive indices of the adhesive layer and optical films to the refractive index of the glass. Regarding JP-A-2002-173656, it is required to further improve the front contrast ratio.